Make drag and drop work on webpage. Also slightly modify drag drop workflow.

ui/aura/client/drag_drop_client.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_AURA_CLIENT_DRAG_DROP_CLIENT_H_
 #define UI_AURA_CLIENT_DRAG_DROP_CLIENT_H_
 #pragma once
 
 #include "ui/aura/aura_export.h"
 #include "ui/aura/event.h"
 
 namespace ui {
 class OSExchangeData;
 }
 
 namespace aura {
 
 class Window;
 
 // An interface implemented by an object that controls a drag and drop session.
 class AURA_EXPORT DragDropClient {
  public:
   virtual ~DragDropClient() {}
 
-  // Initiates a drag and drop session
-  virtual void StartDragAndDrop(const ui::OSExchangeData& data,
+  // Initiates a drag and drop session. Returns the drag operation that was
+  // applied at the end of the drag drop session.
+  virtual int StartDragAndDrop(const ui::OSExchangeData& data,
                                 int operation) = 0;
 
   // Called when mouse is dragged during a drag and drop.
   virtual void DragUpdate(aura::Window* target, const MouseEvent& event) = 0;
 
   // Called when mouse is released during a drag and drop.
   virtual void Drop(aura::Window* target, const MouseEvent& event) = 0;
 
   // Called when a drag and drop session is cancelled.
   virtual void DragCancel() = 0;
+
+  // Returns true if a drag and drop session is in progress.
+  virtual bool IsDragDropInProgress() = 0;
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_CLIENT_DRAG_DROP_CLIENT_H_